1. Field of the Invention
The present invention relates to a semiconductor device having a charge transfer device in which a bipolar transistor and a MOSFET are formed on a single chip.
2. Description of the Related Art
In general, a CCD, which is one type of charge transfer device, is formed on a single chip in combination with n-channel type MOSFETs. The n-channel type MOSFETs, combined with the CCD, constitute a CCD delay line, a CCD drive circuit, a sample/hold circuit, an output circuit, a clock drive circuit, etc. The power source voltage employed in the MOSFET is 12 V or 9 V. Recently, in accordance with a requirement for the reduction in power consumption, a power source voltage of 5 V is employed in order to use MOSFETs in the form of CMOS's, and to match the power source voltage for the FET with that for a bipolar IC formed in another chip.
However, in the case where the power source voltage of 5 V is employed, as compared to the conventionally employed power source voltage of 12 V or 9 V, the input/output linearity of a sample/hold circuit and an operational amplifier such as an output circuit, which are constituted by MOSFETs, is degraded. Thus, the yield of wafers is lowered owing to poor linearity of input/output characteristics in a wafer test.
Also, the only function of a CCD delay line using a CCD is to delay an input signal. The circuit for performing delay processing for this signal is, in general, a bipolar IC formed on a separate chip.
For example, when MOSFETs used in a sample/hold circuit and/or an operational amplifier such as an output circuit are replaced with bipolar transistors which have excellent input/output linearity and operate at high speed at low voltage, and if these bipolar transistors and CCDs are formed on a single chip, the input/output linearity can be improved, the yield is enhanced, and high-performance is achieved. Further, if bipolar transistors for processing signals from CCDs and CCDs are formed on a single chip, such advantages as reduction in size and manufacturing cost, increase in the number of functions, and simplification in system can be brought about.
However, if CCDs are used along with bipolar transistors on a single chip, charge of transfer clocks employed for transferring charge of CCDs may leak to a base region or the like in a bipolar transistor. Because of this, it is difficult to use CCDs along with bipolar transistors on a single chip. In other words, the circuit operation of the bipolar transistor is considerably adversely affected by the leak of transfer clocks, the characteristics of the bipolar transistor are degraded, and the reliability of the product is lowered.